Certain types of marine geophysical surveying, such as seismic or electromagnetic surveying, include towing an energy source at a selected depth in a body of water. One or more geophysical sensor streamers also may be towed in the water at selected depths. The streamers are essentially long cables having geophysical sensors disposed thereon at spaced apart locations. Actuation of the energy source emits an energy field into the body of water. The energy field interacts with the rock formations below the water bottom with changes in the energy field due to this interaction detected by the geophysical sensors positioned on the streamers. The detected energy is used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
For certain types of surveying, it can be important that a sensor streamer is maintained as close as possible to a selected depth profile in the water. For example, a strong source of noise in electromagnetic surveying is induction noise due to streamer movement. Sources of noise in seismic surveying may include vibration, turbulence, and strumming. The noise levels increase when the sensor streamer is unbalanced or exposed to excessive forces, as in the case when the streamer is towed at a slant angle, which may not be the path of least resistance in some instances. Accordingly, the noise levels may be reduced by maintaining the sensor streamer at a substantially constant depth, thereby keeping the streamer as level as possible.
Devices commonly used to regulate streamer depth include depth regulators. In one example, the depth regulators commonly referred to as “birds” may be equipped with variable-incidence wings that are rotatably fixed onto the sensor streamer. The wings allow the birds to be turned about the longitudinal axis of the streamer so that a hydrodynamic force in any given direction about the longitudinal axis of the streamer is obtained. Often, such depth regulators are used in conjunctions with pressure sensors capable of generating a signal related to depth. Typically, pressure sensors used in geophysical surveying can be calibrated to a precision ranging from about 0.1% to about 0.5% of the full scale range of the sensor. The lowest scale pressure sensor used for towed electromagnetic surveying may be in the 20 bar range, for example, resulting in an uncertainty of up to 1 meter in depth. When approaching towing depth of about 500 to about 1,000 meters or greater as can be used in some deep tow applications, pressure sensors in the 100 bar or even 200 bar range may be used having uncertainties in the range of about 5 to about 10 meters. With this uncertainty in the depth measurements, the depth of the depth regulators may be not be substantially constant even when each depth regulator has the same set point, thereby causing increased noise in the geophysical sensor signals.
Accordingly, there is a need for improved methods and systems for controlling depth profile in geophysical surveys such that towing noise may be reduced.